Swain J A, White F N, Peters R M
J Thorac Cardiovasc Surg. 1984 Mar;87(3):445-51.
During cardiac operations using hypothermia, the pH measured at 37 degrees C (and corrected to the patient's body temperature) is generally kept at 7.40. However, ectotherms (cold-blooded animals) regulate pH alkaline of 7.40 as temperature falls, e.g., pH 7.58 at 25 degrees C. Hypothermia and acidosis increase the propensity for ventricular fibrillation (VF). This study was undertaken to determine which scheme of pH regulation during hypothermia provided the highest level of cardiac electrical stability. Eleven dogs underwent median sternotomy and implantation of right ventricular and limb electrodes, aortic and central venous pressure catheters, and a probe to measure pulmonary artery blood temperature. To determine the VF threshold, a programmable stimulator was used to find the minimum current needed to produce VF by sweeping the vulnerable period of the cardiac cycle. The animals were divided into two groups for pH management: eight in the clinical scheme (pH 7.4) and seven in the ectothermic scheme, where pH varied with temperature. Control values were recorded prior to cooling and then repeated when core cooling had lowered the temperature to 25 degrees C. In the clinical group, the VF threshold decreased from 23.1 +/- 4.1 mA at 37 degrees C to 17.0 +/- 3.4 mA at 25 degrees C (p = 0.002); in the ectothermic group, the VF threshold was unchanged by hypothermia (23.7 +/- 3.2 to 22.8 +/- 2.8 mA). Heart rate and arterial and central venous pressures were not different between the groups at either temperature. Corrected arterial pH and PCO2 were unchanged from control in the clinical group at 25 degrees C (pH 7.40 +/- 0.01, arterial PCO2 34 +/- 2 torr), whereas arterial PCO2 was maintained at 20 +/- 1 torr to achieve pH 7.59 +/- 0.01 in the ectothermic group. Five of the eight dogs in the clinical group had spontaneous VF while cooling, as compared to one of the seven dogs in the ectothermic group. These studies demonstrate that allowing the corrected pH to rise with hypothermia and remain alkalotic relative to pH 7.4 improves the electrical stability of the heart during hypothermia, as evidenced by the VF threshold at 25 degrees C. Since the ectothermic scheme increases the electrical stability of the heart, it could decrease the incidence of VF during hypothermia and decrease the temperature at which VF occurs during hypothermic cardiopulmonary bypass.
在使用低温的心脏手术过程中，在37摄氏度测量（并校正至患者体温）的pH值通常保持在7.40。然而，变温动物（冷血动物）会随着温度下降将pH值调节为碱性，高于7.40，例如在25摄氏度时为pH 7.58。低温和酸中毒会增加心室颤动（VF）的倾向。本研究旨在确定低温期间哪种pH调节方案能提供最高水平的心脏电稳定性。11只狗接受了正中胸骨切开术，并植入了右心室和肢体电极、主动脉和中心静脉压力导管以及一个用于测量肺动脉血温的探头。为了确定VF阈值，使用可编程刺激器通过扫描心动周期的易损期来找到产生VF所需的最小电流。将动物分为两组进行pH管理：八只采用临床方案（pH 7.4），七只采用变温方案，即pH随温度变化。在降温前记录对照值，然后在核心体温降至25摄氏度时重复记录。在临床组中，VF阈值从37摄氏度时的23.1±4.1 mA降至25摄氏度时的17.0±3.4 mA（p = 0.002）；在变温组中，低温对VF阈值没有影响（从23.7±3.2 mA降至22.8±2.8 mA）。在两个温度下，两组之间的心率、动脉压和中心静脉压没有差异。在临床组中，25摄氏度时校正后的动脉pH值和PCO2与对照相比没有变化（pH 7.40±0.01，动脉PCO2 34±2托），而在变温组中，动脉PCO2维持在20±1托以达到pH 7.59±0.01。临床组的八只狗中有五只在降温时发生了自发性VF，而变温组的七只狗中有一只发生了自发性VF。这些研究表明，允许校正后的pH值随低温升高并相对于pH 7.4保持碱性可改善低温期间心脏的电稳定性，25摄氏度时的VF阈值证明了这一点。由于变温方案提高了心脏的电稳定性，它可以降低低温期间VF的发生率，并降低低温体外循环期间发生VF的温度。
